Cell and molecular biology, lipoprotein receptors, lipoprotein and cholesterol metabolism, intracellular protein sorting, golgi function, somatic cell genetics, atherosclerosis, scavenger receptors, pathogen receptors, macrophage physiology, pattern recognition in vertebrate and invertebrate immune systems.
We are using genetic, biochemical, physiologic, chemical, cellular and molecular biological methods to study cell surface receptor structure and function. We focus on lipoprotein receptors, in particular the High Density Lipoprotein (HDL) receptor called Scavenger Receptor, Class B, Type I (SR-BI) because of its relevance to many physiological systems and thus to human biology and medicine. The analysis of SR-BI should help provide insights into basic biological processes and contribute to our understanding of atherosclerosis and coronary heart disease (CHD). The risk of developing atherosclerosis is directly related to plasma levels of Low Density Lipoprotein (LDL) cholesterol and inversely related to those of high density lipoprotein (HDL) cholesterol.
Scavenger receptors (SR) are multiligand cell surface receptors defined by their ability to bind modified lipoproteins (e.g., acetylated [Ac] or oxidized [Ox] LDL) with high affinity; however, they bind many other classes of ligands. We have used biochemical and molecular genetic methods to identify and characterize three structurally distinct classes of SR: SR-A, SR-B and SR-C. There are three subtypes of SR-A, which are expressed primarily in mammalian macrophages, and 4 subtypes of SR-C, one of which is expressed in Drosophila melanogaster macrophages. SR-As and SR-CI can both bind a wide variety of polyanions, including chemically modified proteins, some sulfated polysaccharides, the microbial toxins endotoxin and lipoteichoic acid, asbestos, and certain polynucleotides (e.g., poly I, but not poly C). The broad binding specificity of these macrophage scavenger receptors suggest that they may play a role in a variety of macrophage-related physiologic and pathologic systems, e.g., macrophage-associated immune responses (self vs. nonself recognition) and inflammation. Studies of SR-AI/II knockout mice support this suggestion.
SR-BI is an HDL receptor
The SR-Bs, including SR-BI and CD36, are members of the CD36 superfamily of proteins and are expressed on a variety of cell types and tightly bind anionic phospholipids. We cloned SR-BI as an SR and subsequently discovered that it can bind LDL and HDL. Indeed, it was the first physiologically relevant HDL receptor to be described. SR-BI is expressed primarily in liver and steroidogenic tissues. SR-BI, which in some cells is clustered in lipid rafts (e.g., caveolae) rather than coated pits, mediates selective cholesterol uptake from HDL by a mechanism distinct from the classic LDL receptor endocytic pathway.
After HDL binds to SR-BI, cholesterol is transferred to the cells and the cholesterol-depleted particle is released into the extracellular space - there is no lysosomal degradation. This mechanism is called 'selective lipid uptake'. We are using biochemical, biophysical, physiologic, molecular genetic and chemical biological approaches to study the structure and mechanism of action of SR-BI. For example, we have performed large scale, high throughput small molecule screens that have identified potent inhibitors of SR-BI. We have and continue to use these chemical probes combined with structure/activity relationship (SAR) analysis and mutagenesis and biophysical studies to investigate the molecular mechanisms underlying selective lipid uptake.
We have shown that SR-BI expression in vivo is coordinately regulated with cholesterol metabolism (e.g., steroidogeneis) and that genetic manipulation of its expression can significantly influence plasma HDL and biliary cholesterol concentrations. We have also been studying - in a close collaboration with Olivier Kocher at the Harvard Medical School - the mechanism by which the multiple PDZ-domain containing scaffold protein PDKZ1 regulates the activity of hepatic SR-BI. Others have shown that SR-BI is a key, hepatic co-receptor involved in hepatitis C virus infection.
Analysis of mice has established that SR-BI is involved in many mammalian physiologic and pathophysiologic processes, including: maturation, stability and function of blood cells, female fertility, inflammation, thrombosis, responses to infection and apoptosis, movement of lipids into and out of cells, and lipoprotein metabolism, as well as their associated diseases (e.g., atherosclerosis, deep vein thrombosis (DVT), CHD). We have generated SR-BI homozygous null (knockout) mice and exploited their characteristics to generate several novel murine models of fatal occlusive atherosclerotic CHD. These new models appear to be especially attractive for the analysis of the molecular and cellular mechanisms underlying CHD and for developing new pharmacologic and genetic therapies for the prevention and/or treatment of cardiovascular disease. If SR-BI's functions in humans are similar to those in mice, SR-BI will become an attractive target for therapeutic intervention in a variety of diseases.
Exoplasmic cysteine Cys384 of the HDL receptor SR-BI is critical for its sensitivity to a small-molecule inhibitor and normal lipid transport activity. Yu M, Romer KA, Nieland TJ, Xu S, Saenz-Vash V, Penman M, Yesilaltay A, Carr SA, Krieger M. Proc Natl Acad Sci U S A. 2011 Jul 26;108(30):12243-8. Epub 2011 Jul 11.
Identification of the PDZ3 Domain of the Adaptor Protein PDZK1 as a Second, Physiologically Functional Binding Site for the C Terminus of the High Density Lipoprotein Receptor Scavenger Receptor Class B Type I. Kocher O, Birrane G, Yesilaltay A, Shechter S, Pal R, Daniels K, Krieger M. J Biol Chem. 2011 Jul 15;286(28):25171-86. Epub 2011 May 23.
Glycine dimerization motif in the N-terminal transmembrane domain of the high density lipoprotein receptor SR-BI required for normal receptor oligomerization and lipid transport. Gaidukov L, Nager AR, Xu S, Penman M, Krieger M. J Biol Chem. 2011 May 27;286(21):18452-64. Epub 2011 Mar 25.
Negatively cooperative binding of high-density lipoprotein to the HDL receptor SR-BI. Nieland TJ, Xu S, Penman M, Krieger M. Biochemistry. 2011 Mar 22;50(11):1818-30. Epub 2011 Feb 25.